A U.S. Air Force B-2 stealth bomber flies near the Osan Air Base in Pyeongtaek, South Korea, on Thursday. / Shin Young Keun, AP

by Oren Dorell, USA TODAY

by Oren Dorell, USA TODAY

The United States military said Thursday that it flew in two nuclear-capable B-2 bombers to take part in military drills with South Korea after days of bombastic threats from North Korea to turn the U.S. ally into a "sea of fire."

U.S. Strategic Command said the B-2s made the 6,500-mile trip from an air base in Missouri to the Korean Peninsula to show how the United States can conduct long-range precision strikes quickly.

The massive bombers dropped dummy munitions on a South Korean island bombing range and returned home in a single, continuous mission. Known as stealth bombers, they are designed to fly undetected by Soviet-era radar.

The Korean Central News Agency, the mouthpiece of the North Korean state, said the North Korean people were "burning with hatred" over the flights. The United States said the demonstration is meant to show resolve with South Korea.

Defense Secretary Chuck Hagel said Thursday that North Korea's provocative and "bellicose" actions in recent weeks must be taken seriously and dismissed the notion that the Pentagon had overreacted.

"You only need to be wrong once," Hagel said.

Bruce Klingner, a former Korea Branch chief for the CIA, said the B-2 signals to U.S. ally South Korea that the United States intends to fulfill its treaty obligations to defend it if necessary.

"Given North Korea's increasing threats against South Korea and the United States, including threats of nuclear annihilation, this is meant to deter and send a message that the United States is prepared to respond if things get hot," said Klingner, a senior research fellow for Northeast Asia at the Heritage Foundation.

This month, the North Korean Foreign Ministry announced it may attack the United States with nuclear missiles it claims can reach Hawaii and Guam, where the U.S. military has a base. The United States and the United Nations have approved sanctions against the North to get it to end its nuclear program, which is a violation of international agreements signed by the North.

The North also announced recently that it considers void the armistice that ended the Korean War in 1953.

South Korean President Park Geun Hye has said her government will not tolerate any military attacks from the North, which has a long history of launching assaults against previous governments in the South. Park has said she wants to re-engage North Korea, stressing the need for greater trust, but Pyongyang will "pay the price" for any provocation.

In 2010, North Korea sank the South Korean naval corvette Cheonan south of the maritime boundary, killing 46 sailors. That year it attacked Yeonpyeong Island with artillery, killing four South Koreans and destroying 70 homes and buildings.

The reclusive communist dictatorship in the North is led by Kim Jong Un, who took power after his father's death in December 2011.

Scott Snyder, director of the program on U.S.-Korean policy at the Council on Foreign Relations, says the B-2 flights Thursday and B-52 bomber flights last week from Guam also send a signal to China, North Korea's primary ally and sponsor.

"The message to China is that a nuclear North Korea carries with it tangible costs as far as China's regional stability, that this is a problem we can't just let sit and that needs to be addressed and resolved," Snyder said.

At the land border Thursday between the two countries, South Korean soldiers stood at one side of a gate as trucks rumbled through without incident carrying large pipes and containers to Kaesong, an industrial zone in North Korea. Since 2004, the Kaesong factories have operated with South Korean money and know-how; North Korean factory workers are managed by South Koreans.

In recent weeks, North Korea cut other phone and fax hotlines with South Korea's Red Cross and with the American-led U.N. Command at the border. Three other hotlines used to exchange information about air traffic were still operating normally Thursday, according to South Korea's Air Traffic Center.

Despite North Korean rhetoric threatening to unleash its nuclear arsenal, Klingner says recent North Korean troops movements make a more conventional attack in the near future a more likely scenario.

North Korean artillery units have moved closer to its border with South Korea and to five South Korean islands that North Korea has shelled in the past, Klingner said.

"There is a greater risk of miscalculation by Kim, a (greater) likelihood that South Korea would respond militarily to a North Korean attack," Klingner said. Together with military exercises on both sides of the border and everything else that has happened in the past month, "it's generating increasing concern about the situation on the Korean Peninsula," Klingner said.